If you're diving relatively shallow in clear-ish water you can get away without a strobe, but I wouldn't advise it (the onboard flash isn't useful for anything other than triggering strobes in pretty much all Oly housings). To answer your original question about the PT-EP08, assuming it comes with the flat port you can use the following lenses:
9-18 (doable, but not the best)
various Olympus primes like the 25, 30, 45, 75, and probably some Panasonic primes, too
Here's a link to a port chart for the new E-M1II. It uses the same port system, so pretty much everything there applies to the EP08:
For strictly underwater use, I would prefer the original M5 over the M5II just on the basis of the better port system (assuming you're sticking with Oly housings). For example, I use an Inon 170mm dome intended for the old 4/3 system. With that, I can shoot the Oly 12-40 and 7-14 Pro zooms, plus the Panasonic 8-18- all three are too wide to fit in the EP13 housing. In addition, I can also use that dome with the Oly 8mm and the 60mm macro (if I couldn't travel with a flat port).
Off the top of my head, the only underwater advantage of the M5II is better image stabilization, and maybe a slightly better jpeg processor (not an issue if you're shooting RAW). If this is also going to be a topside camera, I might rethink that, but man- those bigger lenses (especially the 12-40) are pretty nice underwater. In my case, i shoot the original M1 underwater, and primarily use my M5II topside. If the housing you're looking at doesn't come with a flat port, you can probably find a used one pretty cheap (I'd probably sell mine for 250 if someone wanted it).